Fluoroelastomers based on vinylidene fluoride (VF.sub.2), such as copolymers with hexafluoropropylene (HFP) and tetrafluoroethylene (TFE), are well-known materials which have a unique combination of properties. Their high chemical, thermal and oxidative stability give them high value in the marketplace. While these elastomers have many desirable properties, their low temperature flexibility is not adequate for all low temperature applications.
There has been considerable effort to improve the low temperature flexibility of these elastomers. It is known that the low temperature flexibility, as measured by the glass transition temperature (T.sub.g), of VF.sub.2 /HFP/TFE copolymers varies inversely with the VF.sub.2 content of the copolymer. However, increasing the VF.sub.2 content of these copolymers is not always desirable, since their chemical resistance becomes poorer as the VF.sub.2 content is raised.
Another method for improving the low temperature flexibility of VF.sub.2 elastomers is to substitute perfluoro(alkyl vinyl ethers) for the HFP in VF.sub.2 /HFP/TFE copolymers. For example, when perfluoro(methyl vinyl ether) (PMVE) is substituted for HFP, the resulting VF.sub.2 /PMVE/TFE copolymers have Tg's which are 10.degree. to 20.degree. C. lower than those of the corresponding VF.sub.2 /HFP/TFE copolymers. While this is a good method for improving the low temperature flexibility of VF.sub.2 fluoroelastomers, the cost of copolymers containing perfluoro (alkyl vinyl ethers) is much higher than that of conventional VF.sub.2 /HFP/TFE copolymers.